1. Field of the Invention
The present invention relates to a flow rate measuring apparatus which is suitable for measuring an intake air amount of an internal combustion engine, for example.
2. Description of the Related Art
As a conventional flow rate measuring apparatus, there is a one which is inserted in a pipe through an apparatus inserting hole formed in the pipe so as to measure the flow rate of fluid flowing in a main passage (see, for example, Japanese Patent Application Laid-Open No. 2009-8619). A connector is formed on the proximal end of the flow rate measuring apparatus so as to protrude to the outside of the pipe, and a measuring passage is formed on the distal end of the same so that a flow rate sensing element is arrange inside the measuring passage. Further, a circuit board with a built-in control circuit is arranged between the connector and the measuring passage.
The measuring passage and the circuit board are inserted inside the pipe, and a part of the fluid flowing in the main passage enters the measuring passage through a flow inlet, flows on a surface of the flow rate sensing element, flows out through a flow outlet, and joins the fluid flowing in the main passage. The flow rate sensing element is provided with a heater, which is supplied with electric current from the circuit to generate heat. Further, the heater contacts the fluid flowing on the surface and is cooled by the fluid so that a variation of resistance corresponding to the flow rate of the fluid is detected.
A fluid temperature sensing element for sensing temperature of the fluid is arranged substantially at the center of the main passage outside the measuring passage at a vicinity of the flow rate sensing element arranged inside the measuring passage. The fluid temperature sensing element is electrically connected by welding to end portions of terminals exposed to the main passage. The other end portions of the terminals also work as connector terminals, and parts other than the both end portions are molded integrally with a base portion.
The conventional flow rate measuring apparatus having the above-mentioned structure can provide the following two effects.
(1) It is possible to prevent the fluid temperature sensing element from causing turbulence by itself in the fluid flowing close to the flow rate sensing element because the fluid temperature sensing element is arranged outside the measuring passage.
(2) It is possible to avoid an influence of heat inside an engine room or heat of the circuit board itself because the fluid temperature sensing element is arranged at substantially the center of the main passage that is apart from the inner wall of the pipe.
However, the conventional technique has the following problem.
The conventional flow rate measuring apparatus has no countermeasure against a break of a wire of the fluid temperature sensing element or deformation or breakage due to an external force when vibration is applied to the pipe.
In other words, in the case of the conventional flow rate measuring apparatus, if vibration is applied to the pipe, the flow rate measuring apparatus is vibrated so that the terminals and the fluid temperature sensing element are also vibrated together. For instance, in the supporting structure of the fluid temperature sensing element as illustrated in FIG. 1 of Japanese Patent Application Laid-Open No. 2009-8619, one of the terminals is longer than the other terminal. Therefore, the vibration of the longer terminal becomes larger than that of the shorter terminal As a result, a bending stress is concentrated on the joint portion between the shorter terminal and the fluid temperature sensing element. A lead wire of the fluid temperature sensing element is usually very thin as a few hundred microns. Therefore, if such a bending stress is applied repeatedly, the lead wire may be broken.
In addition, when the flow rate measuring apparatus is inserted into the pipe through the apparatus inserting hole, the fluid temperature sensing element may contact the wall of the pipe so that the lead wire or the terminal of the fluid temperature sensing element may be bent or broken.
Further, when the flow rate measuring apparatus is handled, a part where the fluid temperature sensing element is arranged may be grabbed by mistake so that the lead wire or the terminal of the fluid temperature sensing element may be bent or broken.